Update documentation for NLP (#594)

* Update Installation.md

website/docs/Installation.md

@@ -37,6 +37,11 @@ pip install flaml[vw]
 pip install flaml[forecast]
 ```
 
+* natural language processing: transformers
+```bash
+pip install flaml[nlp]
+```
+
 ### Distributed tuning
 * ray
 ```bash